This invention relates to a casing arrangement for a marine propulsion unit, such as an outboard motor or the outboard drive portion of an inboard/outboard motor. More particularly, the invention pertains to an improved casing arrangement wherein packing means are provided between adjoining ends of the driveshaft housing and lower unit of the marine propulsion unit to prevent exhaust gases from leaking out from therebetween.
A typical marine propulsion unit, whether of the outboard motor or inboard/outboard motor type, generally includes an internal combustion engine and a lower casing. The lower casing is typically comprised of a driveshaft housing with an expansion chamber formed therein to which exhaust gases are delivered from the exhaust ports of the engine, and a lower unit connected to the driveshaft housing and having a passage for discharging exhaust gases from the chamber to the exterior through an outlet in the propeller.
The lower surface of the driveshaft housing and the upper surface of the lower unit are normally connected by bolts or other suitable connecting means. However, in this arrangement, any seal between the two surfaces and particularly around the exhaust gas discharge passage in the lower unit is effected only by the force of the adjoining surfaces abutting agsint each other. As ar result, engine exhaust gas can leak out of the casing from between the adjoining surfaces of the driveshaft housing and lower unit. In extreme cases, this leaking exhaust gas may corrode the paint layers and ground metal on the driveshaft housing and lower unit, causing the casing to look worn or weathered, and generally degrading the appearance of the propulsion unit. The high temperature of the leaking exhaust gas can also heat up the adjoining surfaces of the driveshaft housing and lower unit and in extreme cases may weaken and soften the surfaces, which in turn, tends to accelerate wear of the surfaces as a result of collisions against each other caused by engine vibration.
It is, therefore, a principal object of this invention to provide an improved casing arrangement which prevents exhaust gas from leaking out from the exhaust gas discharge passage and from between the adjoining surfaces of the driveshaft housing and lower unit.
It is a further object of this invention to prevent the paint layers and ground metal of the casing components from being corroded by leaking exhaust gas from between the mating surfaces of the driveshaft housing and lower unit.
It is yet a further object of this invention to prevent the reduction in pressure durability of the adjoining surfaces of the driveshaft housing and lower unit as a result of leaking exhaust gas from therebetween.